Swainsonine (SW) is an indolizidine alkaloid found in Australian Swainsona canescens (Colegate et al., Aust J Chem 32:2257-2264, 1979), North American plants of the genera Astragalus and Oxytropis (Molyneux R J and James L F., Science 215:190-191, 1981), and also the fungus Rhizoctonia leguminicola (Schneider et al., Tetrahedron 39;29-31, 1983). The alkaloid is a potent inhibitor of the Golgi enzyme .alpha.-mannosidase II, an enzyme required for maturation of N-linked oligosaccharides on newly synthesized glycoproteins. SW also blocks lysosomal .alpha.-mannosidases causing the accumulation of oligomannose chains in cells exposed to the drug (Tulsiani et al., J. Biol. Chem. 257:7936-7939, 1982).
The SW block in Golgi processing prevents expression of the .beta.16GlcNAc branched "complex-type" N-linked oligosaccharides which has been observed to increase following malignant transformation in human and rodent cells (Fernandes et al., Cancer Res. 51:718-723, 1991 and Dennis, J. W., pp. 161-194. CRC Press, Boca Raton, 1991). The branched oligosaccharides appear to play a role in cancer metastasis, as loss or truncation of the structures due to somatic mutations in metastatic tumor cell lines results in greatly reduced metastasis and slower solid tumor growth (Dennis et al., Science 236:582-585, 1987 and VanderElst I. and Dennis J. W., Exp. Cell Res. 192:612-613, 1991). Furthermore, SW-treated murine tumor cells are less metastatic in both organ-colonization and spontaneous metastasis assays in mice (Dennis J. W., Cancer Res. 46:5131-5136, 1986 and Humphries et al., Proc. Natl. Acad. Sci. USA 83:1752-1756, 1986).
SW has been shown to block tumor cell invasion through extracellular matrix in vitro (Yegel et al., Int. J. Cancer 44:685-690, 1989 and Seftor et al., Melanoma Res. 1:53-54, 1991). SW administered either orally or by mini-osmotic pumps to athymic nude mice inhibited the growth rate of human MeWo melanoma and HT29m colon carcinoma tumor xenografts in the mice (Dennis et al., J. Natl. Cancer Inst. 81:1028-1033, 1989 and Dennis et al., Cancer Res., 50:1867-1872, 1990).
The alkaloid has positive effects on cellular immunity in mice (reviewed in Humphries M. J. and Olden K., Pharmacol Ther. 44:85-105, 1989, and Olden et al., Pharmacol Ther 50:285-290, 1991)). In particular, SW has been shown to alleviate both chemically-induced and tumor-associated immune suppression (Hino et al., J. Antibiot. (Tokyo) 38:926-935, 1985), increase NK cell (Humphries et al., Cancer Res. 48:1410-1415, 1988), and LAK cell activities (Yagita M and Saksela E., Scand. J. Immunol. 31:275-282, 1990), and increase splenic and bone marrow (BM) cell proliferation (White et al., Biochem. Biophys. Res. Commun. 150;615-625, 1988; Bowlin et al. Cancer Res 49, 4109-4113, 1989, and White et al., Cancer Commun. 3:83-91, 1991). SW has also been shown to be hemorestorative in mice following treatment with both cycle-specific and nonspecific chemotherapeutic agents (Oredipe et al., J. Natl. Cancer Inst. 83:1149-1156, 1991).
Japanese Patent Application No. J61277685 describes indolizidine derivatives which are reported to be useful as immune regulators, which can be administered orally or parenterally at a dose of about 0.1-100 ml/kg a day. It is also reported that the indolizidine derivatives may be used in combination with antitumour agents, antimicrobial agents or antiinflammatories.
Carbonoyloxy substitutions at the 2 and 8 carbons of swainsonine have been reported to reduce inhibitor activity by 2-3 orders of magnitude for Jack Bean and MDAY-D2 tumor cell lysosomal mannosidases in vitro. However, 2-p-nitrobenzoyloxy, 2-octanoyloxy- and 2-butanoyloxy-derivatives of swainsonine retained full activity as inhibitors of Golgi oligosacccharide processing in viable MDAY-D2 tumor cells. Inhibition of oligosaccharide processing was reduced by the esterase inhibitor diethyl p-nitrophenyl phosphate, suggesting that while the compounds are relatively poor inhibitors of mannosidase in vitro, the compounds enter cells at a rate comparable to that of SW and are converted to SW by cellular esterases. The more lipophilic esters, 2-benzoyloxy-SW, 2-toluoyloxy-SW, 8-palmitoyloxy-SW and 8-myristinoyloxy-SW, showed IC.sub.50 values at least 10 times higher for inhibition of Golgi oligosaccharide processing, probably due to less efficient entry of the compounds into tumor cells. The anti-metastatic activities of SW and two analogs were tested and shown to correlate with the IC.sub.50 values for inhibition of Golgi oligosaccharide processing in cultured tumor cells. In vivo, SW and the analogs were administered intraperitoneally to mice and found to have comparable activities as stimulators of bone marrow cell proliferation. (Dennis, J. W. et al. Biochemical Pharmacology 46:1459-1466, 1993).